Conventionally, the measurement of the amount of an analyte in a sample using a redox reaction has been utilized for a wide range of applications. For example, such measurement is carried out in the following manner. First, a peroxidase (hereinafter referred to as “POD”) and a reducing agent are added to an oxidizing substance as an analyte or to an oxidizing substance formed from an analyte, so that a redox reaction occurs between the oxidizing substance and the reducing agent with the POD as a catalyst. When a reducing agent that develops color when it is oxidized is used as the reducing agent, the amount of the oxidizing substance can be determined by measuring the color developed because there is a correlation between the amount of the color developed and the amount of the oxidizing substance. As the reducing agent that develops color when it is oxidized, N-(carboxymethylaminocarbonyl)-4,4′-bis(dimethylamino)diphenylamine sodium salt has been used.
However, such a method does not exhibit sufficient measurement sensitivity and thus may fail to improve the accuracy of the measurement.